The present invention relates generally to computed tomography (CT) imaging systems and more particularly, to a system for maintaining bearing temperatures of an anode as well as minimizing focal spot displacement due to thermal expansion of anode related components.
A CT imaging system typically includes a gantry that rotates at various speeds in order to create a 360° image. The gantry contains an x-ray source, such as an x-ray tube that generates x-rays across a vacuum gap between a cathode and an anode. The anode has a target that is coupled to a stem, which rotates on a pair of anode bearings. X-rays are emitted from the target and are projected in the form of a fan-shaped beam, which is collimated to lie within an X-Y plane of a Cartesian coordinate system, generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile for generation of an image.
It is desirable to increase gantry rotating speeds and CT tube peak operating power such that quicker imaging times and improved image quality can be provided. In order to do so certain requirements must be satisfied, such as the ability to operate the anode bearings within a wide range of the power spectrum, i.e. approximately 0–8 kw. However, the dry lubrication typically used in the bearings has an optimal operating temperature range of approximately 400° C.–550° C. Large fluctuations in power spectrum operation can result if the bearings are operated outside this temperature range.
Also, it is further required that focal spot displacement, in the anode axial direction, should be minimized during operation of a CT system. Thermal expansion of the stem and other anode related components, however, can cause the position of the target to change and thus the location of the focal spot to change. This focal spot displacement can negatively affect performance of a CT system.
Current anode designs are unable to satisfy the above-described requirements. Thus, there exists a need for an improved CT system that maintains bearing operating temperature of an anode within a desired operating range and minimizes focal spot displacement of that anode.